The present invention relates to keyboard encoder circuitry in general and particularly to keyboard encoding circuits for developing different predetermined analog voltage levels for each keyboard switch closure and utilizing an analog-to-digital converter for obtaining a digital output code for each voltage level.
Keyboard encoder circuits typically transmit a binary digital code to a data acquisition device such as a computer in response to closure of a key actuated switch, the code transmitted being unique for each key. Parallel encoding circuits typically require one data wire connection between the keyboard and the computer for each bit in the binary code. Serial type signal encoders typically reduce the number of data wires required to two by sending the code sequentially.
Often the encoder circuits, whether of the parallel or serial type, produce radio frequency noise which can be detrimental in some environments. This radio frequency noise can be avoided if the keyboard output signal comprises a low level analog voltage signal rather than a conventional digital signal. In U.S. Pat. No. 4,015,254 (Strandt) a keyboard encoding circuit is disclosed comprising a plurality of keyboard switches each having one side connected with a multi-resistor voltage divider network and a second side connected in common. With a DC voltage applied to the network, a unique keyboard output voltage is produced on the common switch connection for each switch closure. The common connection is coupled to a remote analog-to-digital converter which converts the unique voltage to a unique digital code for use by a computer.
One drawback to the encoding circuit described by Strandt lies in the inability of the circuit to determine when two or more keys have been simultaneously actuated. When two keys are actuated, the voltage associated with one of the keys is transmitted to the A/D converter. It would be beneficial in many applications if the simultaneous activation of two or more keys would produce an output voltage that is different than the output voltage produced by activating any one key.
Another drawback of the Strandt encoding circuit lies in the number of resistors required. Each additional key switch requires an additional resistor in the voltage divider network increasing the cost and size of the encoder. It would be of further benefit if the number of resistors required could be reduced particularly with respect to keyboards incorporating a large number of keys.